my %entries;
while ( my $entry = <DATA> ) {
    chomp $entry;
    my ( $size, $name ) = split /\s+/, $entry;
    push @{$entries{$size}}, $name;
}

local $" = "\t";
print scalar( @{$entries{$_}} ), ": @{$entries{$_}}\n" for
    sort { $a <=> $b } keys %entries;

__DATA__
12 filename1
14 filename2
14 filename3
11 filename4
14 filename5